Undersea fiber optic communication systems may include a main trunk path extending between land-based cable stations. The main trunk is defined by an undersea cable having a plurality of optical fibers therein and one or more repeaters or optical amplifiers disposed along the trunk path used to amplify optical signals transmitted between the cable stations. Each cable station includes terminal equipment used to transmit and receive these optical signals along the main trunk path. Such undersea systems may also include one or more branch segments which are coupled to the trunk by a branching unit (BU). A branch segment extends from the branching unit connected along the main trunk to a branch segment cable station. Trunk cable stations may be used to carry information signals through the backbone of the network while branch cable stations may be used to add or drop traffic from the trunk path. The optical signals transmitted between the cable stations are typically dense wavelength-division multiplexed (DWDM) signals in which a plurality of optical channels, each at a respective wavelength, are multiplexed together.
FIG. 1 illustrates a conventional and simplified trunk and branch network configuration including trunk 110 disposed between cable stations A and B and a branch segment 120 connecting cable station C to trunk 110 via optical add-drop multiplexer (OADM) node 100. In general, an OADM node is used to add and/or drop channels within a DWDM optical signal between the cable stations and may be implemented in one or more separate units or bodies which are connected via cabled fibers. Again, trunk path 110 is defined by an optical cable having a plurality of optical fiber pairs 115, one or more optical amplifiers disposed along the optical cable as well as other optical/electrical equipment used to transmit optical signals from cable station A to B (A-B) and/or from cable station B to A (B-A). Typically, the optical signals or “through traffic” travel along trunk 110 between cable stations A and B, whereas signals destined for cable station C are added/dropped from the trunk 110 using OADM 100 and supplied to branch segment 120. For each fiber pair 115 along trunk 110 there are two corresponding fiber pairs 125 and 130 within branch segment 120 in order to provide transmission capacity in both directions from OADM 100 to cable station C thereby supporting connectivity between all three cable stations A, B and C. In particular, if trunk 110 includes eight (8) fiber pairs, and all fiber pairs on the trunk support connectivity of cable stations A and B with cable station C, then branch segment 120 would include sixteen (16) fiber pairs to accommodate transmission to/from cable station C. In the current state of the art, if one branch fiber pair per trunk fiber pair is used, then it is not possible to have connectivity between each combination of two of the three end stations. Therefore, given one trunk fiber pair and one branch fiber pair there can be either connectivity between A and C (A-C) and between A and B (A-B); or connectivity between B and C (B-C) and between A and B (A-B); but there cannot be connectivity of all three A-C, A-B and B-C on the trunk fiber pair.
Because the undersea cable used to connect cable station C to OADM 100 includes twice the number of fiber pairs as compared to the cable for trunk path 110, the cable and the associated optical repeaters manufactured for branch segment 120 will be more costly to support the higher number of fiber paths and optical amplifiers respectively. In addition, the spectral efficiency of branch segment 120 will be low compared to the spectral efficiency of trunk path 110 since multiple fiber pairs are required for bidirectional traffic. In particular, spectral efficiency is the information rate transmitted or total transmission capacity over a given bandwidth within a communication network or in this case, branch segment 120. In other words, by more efficiently using available spectral bandwidth the greater the spectral efficiency. For the branch segment 120, one of the fiber pairs (e.g. 130) accommodates traffic to/from OADM 100 from/to cable station C and the other fiber pair (e.g. 125) accommodates traffic to/from cable station C from/to OADM 100. Since there are separate fiber pairs to accommodate directional traffic, spectral efficiency for each of the two fiber pairs 125 and 130 in the branch is lower as compared to the spectral efficiency of the corresponding trunk fiber pair 115. In the conventional optical networks, if a one to one correspondence of fiber pairs in the branch segment to the trunk path is used, then connectivity between each combination of two of the three end stations A, B and C is not possible. For example, in conventional exemplary networks, fiber pair 130 in branch segment 120 would be exclusively dedicated to bidirectional communication between cable stations B and C, and fiber pair 125 in branch segment 120 would be exclusively dedicated to bidirectional communication between cable stations A and C. Therefore if fiber pair 130 (alternatively 125) is not implemented in branch segment 120 then the corresponding connectivity between B and C (alternatively A and C) will not be present in OADM node 100. Therefore, given one trunk fiber pair and one branch fiber pair, in a conventional optical network there can be either connectivity between A and C (A-C) and between A and B (A-B) if branch fiber pair 125 is implemented and branch fiber 130 is not implemented; or connectivity between B and C (B-C) and between A and B (A-B) if branch fiber pair 130 is implemented and branch fiber pair 125 is not implemented. However, both fiber pairs 125 and 130 have to be implemented in branch 120 to allow for all three of A-C, A-B and B-C connections via OADM node 100 to be present as shown in FIG. 1. Thus, a need exists for an OADM node that allows for symmetric fiber pairs to be used. That is, for each fiber pair in the trunk, only a single corresponding fiber pair is needed in a branch to enable connectivity between each combination of two of three end stations. It is with respect to these and other considerations that the present improvements have been needed.